Method for producing fermented food, fermented food, and lactic acid bacteria-containing composition

ABSTRACT

The present invention provides a lactic acid bacteria-containing composition comprising a  Lactobacillus delbrueckii  and a  Streptococcus thermophilus.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a method for producing a fermented food, a fermented food, and a lactic acid bacteria-containing composition, and more particularly to a method for producing a fermented food, a lactic acid bacteria-containing composition used therein, and a fermented food obtained therefrom.

Related Background Art

Examples of the fermented food include fermented milk defined as the “products which are obtained by fermenting milk, or milk, etc. containing an equal or greater amount of milk solids-not-fat with lactic acid bacteria or yeast and then forming a paste or liquid, or the frozen product” in the Japanese “Ministerial Ordinance on Milk and Milk products Concerning Compositional Standards, etc.” Representative examples of such fermented milk include yogurt such as set type yogurt (solid fermented milk), soft type yogurt (pasty fermented milk), and drink type yogurt (liquid fermented milk).

In the production of yogurt, for example, a Lactobacillus species and Streptococcus thermophilus are used as lactic acid bacteria. In recent years, the mainstream yogurt has been prepared by fermenting raw material milk inoculated with a combination of Lactobacillus delbrueckii subsp. bulgaricus (L. delbrueckii subsp. bulgaricus, L. bulgaricus) and Streptococcus thermophilus (S. thermophilus). Such yogurt mainly has characteristics of a refreshing acidity and a fermented aroma.

For example, International Publication No. WO2018/151249 (Patent Document 1) describes a fermented milk production method comprising a step for adding lactic acid bacteria starter to a raw material mix to obtain a fermented milk base material; and a fermentation step for fermenting the fermented milk base material at 35 to 50° C., wherein the lactic acid bacteria starter includes L. bulgaricus and S. thermophilus. Moreover, International Application Japanese-Phase Publication No. 2015-518374 (Patent Document 2) describes the use of a certain Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus for the production of fermented milk products.

In addition, it is possible to prepare a fermented food even in the case of using bacteria whose species are same as L. delbrueckii subsp. bulgaricus but whose subspecies is different from L. delbrueckii subsp. bulgaricus. However, in the production of a fermented food, the use of bacteria whose species are same as L. delbrueckii subsp. bulgaricus but whose subspecies are different from L. delbrueckii subsp. bulgaricus makes it difficult to stably produce a fermented food as compared with the case of using L. delbrueckii subsp. bulgaricus, causing a problem that fermentation is impossible, a fermented food can be prepared but fermentation takes too long, or the flavor is not good depending on the bacterial species used (different subspecies).

SUMMARY OF THE INVENTION

In addition, the present inventors have examined a method for producing a fermented food using a Lactobacillus species and Streptococcus thermophilus, and have found a problem that prtS (−) S. thermophilus carrying no prtS gene, which have been conventionally used for the production of fermented milk (mainly yogurt), cannot be fermented or take too much time for fermentation when used in combination with a Lactobacillus delbrueckii species which cannot utilize lactose as an energy source, that is, which has no lactose utilization.

In addition, conventionally, yogurt having a refreshing acidity and fermented aroma as described above was the mainstream. However, in recent years, demands for a balanced and mild flavor in all of acidity, umami, fermented aroma, and richness have been increasing due to diversification of consumer preferences and yogurt eating methods.

The present invention aims to provide a method for producing a fermented food using a Lactobacillus species and Streptococcus thermophilus, a lactic acid bacterium and a lactic acid bacteria-containing composition which can be used therein, and a fermented food obtained therefrom. The method for producing a fermented food makes it possible to produce a fermented food even when using Lactobacillus delbrueckii which has no lactose utilization and to obtain a fermented food having a balanced and mild flavor in all of acidity, umami, fermented aroma, and richness.

The present inventors have made earnest studies to achieve the above object, and have found as a result that, among Streptococcus thermophilus, many strains derived from Japanese raw milk carry the prtS gene. Streptococcus thermophilus carrying the prtS gene can be fermented in the case of a single strain, and moreover, when this is used in combination with Lactobacillus delbrueckii for fermentation of raw material milk, the time required for fermentation can be significantly reduced. Furthermore, the present inventors have found that, with Streptococcus thermophilus carrying the prtS gene, a fermented food can be prepared in a short time even when the Lactobacillus delbrueckii has no lactose utilization.

What is more, the present inventors have found that it is possible to stably obtain a fermented food having a balanced and mild flavor particularly in all of acidity, umami, fermented aroma, and richness when the Streptococcus thermophilus carrying the prtS gene is combined with a specific Lactobacillus delbrueckii classified into a cluster different from the conventional Lactobacillus delbrueckii by multilocus sequence analysis (MLSA) classification. Thus, the present invention has been completed.

Specifically, the present invention provides the following.

-   -   [1] A method for producing a fermented food, comprising: a         fermentation step of fermenting a raw material milk-containing         milk preparation solution added with Lactobacillus delbrueckii         and Streptococcus thermophilus carrying a prtS gene.     -   [2] The method for producing a fermented food according to [1],         wherein the Lactobacillus delbrueckii is Lactobacillus         delbrueckii classified into any one of clusters I, II, III, and         V by MLSA classification based on seven housekeeping genes         consisting of fusA gene, gyrB gene, hsp60 gene, ileS gene, pyrG         gene, recA gene, and recG gene.     -   [3] The method for producing a fermented food according to [1]         or [2], wherein the Lactobacillus delbrueckii is at least one         selected from the group consisting of Lactobacillus delbrueckii         satisfying all of the following conditions (i) to (vii):     -   (i) carrying an fusA gene whose allele number is 2 or 14,     -   (ii) carrying a gyrB gene whose allele number is any one         selected from 3, 17, 18, and 25,     -   (iii) carrying an hsp60 gene whose allele number is any one         selected from 4, 18, 26, and 27,     -   (iv) carrying an ileS gene whose allele number is any one         selected from 16, 20, 21, 30, and 33,     -   (v) carrying a pyrG gene whose allele number is 22 or 23,     -   (vi) carrying an recA gene whose allele number is any one         selected from 2, 7, 24, and 28, and     -   (vii) carrying an recG gene whose allele number is 3, and         Lactobacillus delbrueckii classified into the same cluster as         that of the Lactobacillus delbrueckii satisfying all of the         conditions (i) to (vii) by MLSA classification based on seven         housekeeping genes consisting of fusA gene, gyrB gene, hsp60         gene, ileS gene, pyrG gene, recA gene, and recG gene.     -   [4] The method for producing a fermented food according to any         one of [1] to [3], wherein the Lactobacillus delbrueckii is at         least one selected from the group consisting of Lactobacillus         delbrueckii specified by accession number NITE BP-02874, and         Lactobacillus delbrueckii classified into the same cluster as         that of the Lactobacillus delbrueckii specified by accession         number NITE BP-02874 by MLSA classification based on seven         housekeeping genes consisting of fusA gene, gyrB gene, hsp60         gene, ileS gene, pyrG gene, recA gene, and recG gene.     -   [5] The method for producing a fermented food according to any         one of [1] to [4], wherein the Lactobacillus delbrueckii is         Lactobacillus delbrueckii specified by accession number NITE         BP-02874.     -   [6] The method for producing a fermented food according to [1]         or [2], wherein the Lactobacillus delbrueckii is Lactobacillus         delbrueckii which has no lactose utilization.     -   [7] The method for producing a fermented food according to any         one of [1] to [6], wherein the Streptococcus thermophilus         carrying the prtS gene is Streptococcus thermophilus specified         by accession number NITE BP-02875.     -   [8] A fermented food comprising Lactobacillus delbrueckii and         Streptococcus thermophilus carrying a prtS gene.     -   [9] The fermented food according to [8], wherein the         Lactobacillus delbrueckii is Lactobacillus delbrueckii         classified into any one of clusters I, II, III, and V by MLSA         classification based on seven housekeeping genes consisting of         fusA gene, gyrB gene, hsp60 gene, ileS gene, pyrG gene, recA         gene, and recG gene.     -   [10] The fermented food according to [8] or [9], wherein the         Lactobacillus delbrueckii is at least one selected from the         group consisting of Lactobacillus delbrueckii satisfying all of         the following conditions (i) to (vii):     -   (i) carrying an fusA gene whose allele number is 2 or 14,     -   (ii) carrying a gyrB gene whose allele number is any one         selected from 3, 17, 18, and 25,     -   (iii) carrying an hsp60 gene whose allele number is any one         selected from 4, 18, 26, and 27,     -   (iv) carrying an ileS gene whose allele number is any one         selected from 16, 20, 21, 30, and 33,     -   (v) carrying a pyrG gene whose allele number is 22 or 23,     -   (vi) carrying an recA gene whose allele number is any one         selected from 2, 7, 24, and 28, and     -   (vii) carrying an recG gene whose allele number is 3, and         Lactobacillus delbrueckii classified into the same cluster as         that of the Lactobacillus delbrueckii satisfying all of the         conditions (i) to (vii) by MLSA classification based on seven         housekeeping genes consisting of fusA gene, gyrB gene, hsp60         gene, ileS gene, pyrG gene, recA gene, and recG gene.     -   [11] The fermented food according to anyone of [8] to [10],         wherein the Lactobacillus delbrueckii is at least one selected         from the group consisting of Lactobacillus delbrueckii specified         by accession number NITE BP-02874, and Lactobacillus delbrueckii         classified into the same cluster as that of the Lactobacillus         delbrueckii specified by accession number NITE BP-02874 by MLSA         classification based on seven housekeeping genes consisting of         fusA gene, gyrB gene, hsp60 gene, ileS gene, pyrG gene, recA         gene, and recG gene.     -   [12] The fermented food according to any one of [8] to [11],         wherein the Lactobacillus delbrueckii is Lactobacillus         delbrueckii specified by accession number NITE BP-02874.     -   [13] The fermented food according to [8] or [9], wherein the         Lactobacillus delbrueckii is Lactobacillus delbrueckii which has         no lactose utilization.     -   [14] The fermented food according to anyone of [8] to [13],         wherein the Streptococcus thermophilus carrying the prtS gene is         Streptococcus thermophilus specified by accession number NITE         BP-02875.     -   [15] Streptococcus thermophilus specified by accession number         NITE BP-02875.     -   [16] The Streptococcus thermophilus specified by accession         number NITE BP-02875 according to [15], which is the         Streptococcus thermophilus carrying the prtS gene and used in         the method for producing a fermented food according to any one         of [1] to [7].     -   [17] At least one Lactobacillus delbrueckii selected from the         group consisting of Lactobacillus delbrueckii satisfying all of         the following conditions (i) to (vii):     -   (i) carrying an fusA gene whose allele number is 2 or 14,     -   (ii) carrying a gyrB gene whose allele number is any one         selected from 3, 17, 18, and 25,     -   (iii) carrying an hsp60 gene whose allele number is any one         selected from 4, 18, 26, and 27,     -   (iv) carrying an ileS gene whose allele number is any one         selected from 16, 20, 21, 30, and 33,     -   (v) carrying a pyrG gene whose allele number is 22 or 23,     -   (vi) carrying an recA gene whose allele number is any one         selected from 2, 7, 24, and 28, and     -   (vii) carrying an recG gene whose allele number is 3, and         Lactobacillus delbrueckii classified into the same cluster as         that of the Lactobacillus delbrueckii satisfying all of the         conditions (i) to (vii) by MLSA classification based on seven         housekeeping genes consisting of fusA gene, gyrB gene, hsp60         gene, ileS gene, pyrG gene, recA gene, and recG gene.     -   [18] The Lactobacillus delbrueckii according to [17], which is         at least one selected from the group consisting of Lactobacillus         delbrueckii specified by accession number NITE BP-02874, and         Lactobacillus delbrueckii classified into the same cluster as         that of the Lactobacillus delbrueckii specified by accession         number NITE BP-02874 by MLSA classification based on seven         housekeeping genes consisting of fusA gene, gyrB gene, hsp60         gene, ileS gene, pyrG gene, recA gene, and recG gene.     -   [19] The Lactobacillus delbrueckii according to [17] or     -   [18], which is the Lactobacillus delbrueckii used in the method         for producing a fermented food according to any one of [1] to         [7].     -   [20] A lactic acid bacteria-containing composition used in the         method for producing a fermented food according to any one of         [1] to [7], comprising: Lactobacillus delbrueckii and/or         Streptococcus thermophilus carrying a prtS gene.     -   [21] The lactic acid bacteria-containing composition according         to [20], wherein the Streptococcus thermophilus is the         Streptococcus thermophilus according to [15] or [16].     -   [22] The lactic acid bacteria-containing composition according         to [20] or [21], wherein the Lactobacillus delbrueckii is the         Lactobacillus delbrueckii according to any one of [17] to [19].

The present invention makes it possible to provide a method for producing a fermented food using a Lactobacillus species and Streptococcus thermophilus, a lactic acid bacterium and a lactic acid bacteria-containing composition which can be used therein, and a fermented food obtained therefrom. The method for producing a fermented food makes it possible to produce a fermented food even when using Lactobacillus delbrueckii which has no lactose utilization and to obtain a fermented food having a balanced and mild flavor in all of acidity, umami, fermented aroma, and richness.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a phylogenetic tree obtained by performing MLSA classification on 61 strains of Lactobacillus delbrueckii;

FIG. 2A is a diagram illustrating the allele number of the fusA gene and its base sequence (bases 1 to 200);

FIG. 2B is a diagram illustrating the allele number of the fusA gene and its base sequence (bases 201 to 400);

FIG. 2C is a diagram illustrating the allele number of the fusA gene and its base sequence (bases 401 to 474);

FIG. 3A is a diagram illustrating the allele number of the gyrB gene and its base sequence (bases 1 to 100);

FIG. 3B is a diagram illustrating the allele number of the gyrB gene and its base sequence (bases 101 to 200);

FIG. 3C is a diagram illustrating the allele number of the gyrB gene and its base sequence (bases 201 to 300);

FIG. 3D is a diagram illustrating the allele number of the gyrB gene and its base sequence (bases 301 to 380);

FIG. 4A is a diagram illustrating the allele number of the hsp60 gene and its base sequence (bases 1 to 100);

FIG. 4B is a diagram illustrating the allele number of the hsp60 gene and its base sequence (bases 101 to 200);

FIG. 4C is a diagram illustrating the allele number of the hsp60 gene and its base sequence (bases 201 to 300);

FIG. 4D is a diagram illustrating the allele number of the hsp60 gene and its base sequence (bases 301 to 400);

FIG. 4E is a diagram illustrating the allele number of the hsp60 gene and its base sequence (bases 401 to 500);

FIG. 4F is a diagram illustrating the allele number of the hsp60 gene and its base sequence (bases 501 to 511);

FIG. 5A is a diagram illustrating the allele number of the ileS gene and its base sequence (bases 1 to 100);

FIG. 5B is a diagram illustrating the allele number of the ileS gene and its base sequence (bases 101 to 200);

FIG. 5C is a diagram illustrating the allele number of the ileS gene and its base sequence (bases 201 to 300);

FIG. 5D is a diagram illustrating the allele number of the ileS gene and its base sequence (bases 301 to 400);

FIG. 5E is a diagram illustrating the allele number of the ileS gene and its base sequence (bases 401 to 496);

FIG. 6A is a diagram illustrating the allele number of the pyrG gene and its base sequence (bases 1 to 100);

FIG. 6B is a diagram illustrating the allele number of the pyrG gene and its base sequence (bases 101 to 200);

FIG. 6C is a diagram illustrating the allele number of the pyrG gene and its base sequence (bases 201 to 300);

FIG. 6D is a diagram illustrating the allele number of the pyrG gene and its base sequence (bases 301 to 392);

FIG. 7A is a diagram illustrating the allele number of the recA gene and its base sequence (bases 1 to 100);

FIG. 7B is a diagram illustrating the allele number of the recA gene and its base sequence (bases 101 to 200);

FIG. 7C is a diagram illustrating the allele number of the recA gene and its base sequence (bases 201 to 300);

FIG. 7D is a diagram illustrating the allele number of the recA gene and its base sequence (bases 301 to 400);

FIG. 7E is a diagram illustrating the allele number of the recA gene and its base sequence (bases 401 to 412);

FIG. 8A is a diagram illustrating the allele number of the recG gene and its base sequence (bases 1 to 100);

FIG. 8B is a diagram illustrating the allele number of the recG gene and its base sequence (bases 101 to 200);

FIG. 8C is a diagram illustrating the allele number of the recG gene and its base sequence (bases 201 to 300);

FIG. 8D is a diagram illustrating the allele number of the recG gene and its base sequence (bases 301 to 400);

FIG. 8E is a diagram illustrating the allele number of the recG gene and its base sequence (bases 401 to 490);

FIG. 9 is a graph illustrating the fermentation times when using S. thermophilus prtS(−) and when using S. thermophilus prtS(+);

FIG. 10 is a graph illustrating the fermentation times when using L. delbrueckii Lac(−) and when using L. delbrueckii Lac(+);

FIG. 11 is a diagram illustrating the relationship between the first principal component score (PC1) and the second principal component score (PC2) in the analysis result of water-soluble component (1);

FIG. 12 is a diagram illustrating the relationship between the first principal component score (PC1) and the second principal component score (PC2) in the analysis result of water-soluble component (1);

FIG. 13 is a diagram illustrating the relationship between the first principal component score (PC1) and the third principal component score (PC3) in the analysis result of water-soluble component (1);

FIG. 14 is a diagram illustrating the relationship between the second principal component score (PC2) and the third principal component score (PC3) in the analysis result of water-soluble component (1);

FIG. 15 is a diagram illustrating the relationship between the first principal component score (PC1) and the second principal component score (PC2) in the analysis result of water-soluble component (2);

FIG. 16 is a scatter diagram of the water-soluble components corresponding to FIG. 15 in the analysis result of water-soluble component (2);

FIG. 17 is a diagram illustrating the relationship between the first principal component score (PC1) and the second principal component score (PC2) in the analysis result of aroma component (1);

FIG. 18 is a diagram illustrating the relationship between the first principal component score (PC1) and the second principal component score (PC2) in the analysis result of aroma component (2);

FIG. 19 is a scatter diagram of the aroma components corresponding to FIG. 18 in the analysis result of aroma component (2);

FIG. 20 is a diagram illustrating the relationship between the first principal component score (PC1) and the second principal component score (PC2) when using S. thermophilus prtS(−) in the sensory evaluation result (1);

FIG. 21 is a scatter diagram of the sensory evaluation items corresponding to FIG. 20 in the sensory evaluation result (1);

FIG. 22 is a diagram illustrating the relationship between the first principal component score (PC1) and the second principal component score (PC2) when using S. thermophilus prtS(+) in the sensory evaluation result (1);

FIG. 23 is a scatter diagram of the sensory evaluation items corresponding to FIG. 22 in the sensory evaluation result (1);

FIG. 24 is a diagram illustrating the relationship between the first principal component score (PC1) and the second principal component score (PC2) in the sensory evaluation result (2);

FIG. 25 is a scatter diagram of the sensory evaluation items corresponding to FIG. 24 in the sensory evaluation result (2);

FIG. 26 is a graph illustrating the relationship between the amount of D-lactic acid and the acidity score in each type of fermented milk which results from a measurement of lactic acid (1);

FIG. 27 is a graph illustrating the relationship between the amount of L-lactic acid and the acidity score in each type of fermented milk which results from a measurement of lactic acid (1);

FIG. 28 is a graph illustrating the relationship between the ratio of D-lactic acid and the acidity score in each type of fermented milk which results from a measurement of lactic acid (1);

FIG. 29 is a graph illustrating the ratio of D-lactic acid in each type of fermented milk when using S. thermophilus prtS(−) and when using S. thermophilus prtS(+), each for the case of using L. delbrueckii classified into clusters I to V (all) which results from a measurement of lactic acid (2);

FIG. 30 is a graph illustrating the ratio of D-lactic acid in each type of fermented milk when using S. thermophilus prtS(−) and when using S. thermophilus prtS(+), each for the case of using L. delbrueckii classified into clusters I, II, III, V (except for IV) which results from a measurement of lactic acid (2);

FIG. 31 is a graph illustrating fermentation times when using S. thermophilus prtS(−) and when using S. thermophilus prtS(+) in the examination on S. thermophilus strains (1); and

FIG. 32 is a graph illustrating the ratio of D-lactic acid in each type of fermented milk when using S. thermophilus prtS(−) and when using S. thermophilus prtS(+) the examination on S. thermophilus strains(2).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention is described in detail with reference to preferred embodiments.

<Method for Producing Fermented Food>

The method for producing a fermented food of the present invention comprises a fermentation step of fermenting a raw material milk-containing milk preparation solution added with Lactobacillus delbrueckii and Streptococcus thermophilus carrying a prtS gene.

(Milk Preparation Solution)

The milk preparation solution according to the present invention contains raw material milk. Preferably, the raw material milk contains lactose, and examples thereof include raw milk (for example, milk of cows, buffaloes, sheep, goats, and the like), sterilized milk, full-fat milk, skim milk, whey, and processed products thereof (such as whole fat milk powder, whole fat concentrated milk, skimmed milk powder, defatted concentrated milk, condensed milk, whey powder, buttermilk, butter, cream, cheese, whey protein concentrate (WPC), whey protein isolate (WPI), α-lactalbumin (α-La), and β-lactoglobulin (β-Lg)). One of these or a mixture of two or more thereof may be used.

The milk preparation solution according to the present invention may be composed of only the raw material milk, may be an aqueous solution, diluent, or concentrated liquid of the raw material milk, or may further contain other components, if necessary, in addition to the raw material milk. Examples of the other components include water; soy milk, saccharides including sugar, sweeteners, flavors, fruit juices, fruit pulp, vitamins, minerals, oils and fats, ceramides, collagen, milk phospholipids, polyphenols, and other foods, food ingredients, and food additives; and stabilizers such as pectin, soybean polysaccharide, carboxy methylcellulose (CMC), agar, gelatin, carrageenan, and gums, thickeners, and gelling agents. One of these or a mixture of two or more thereof may be used. The milk preparation solution can be prepared by mixing the above components while heating as necessary and/or homogenizing as necessary. In addition, as the above-mentioned milk preparation solution, a solution sterilized by heating can be used.

(Lactobacillus Delbrueckii)

The method for producing a fermented food of the present invention ferments the above-described milk preparation solution added with a combination of Lactobacillus delbrueckii and Streptococcus thermophilus carrying the following prtS gene.

Lactobacillus delbrueckii (hereinafter sometimes referred to as “L. delbrueckii”) is a bacterial species classified into the genus Lactobacillus. It is known that there are six subspecies of Lactobacillus delbrueckii subsp. bulgaricus, Lactobacillus delbrueckii subsp. delbrueckii, Lactobacillus delbrueckii subsp. lactis, Lactobacillus delbrueckii subsp. indicus, Lactobacillus delbrueckii subsp. sunkii, and Lactobacillus delbrueckii subsp. jakobsenii in the Lactobacillus delbrueckii.

In addition, L. delbrueckii includes L. delbrueckii which can catabolize lactose into glucose and galactose for utilize as an energy source, that is, which has lactose utilization (hereinafter sometimes referred to as “L. delbrueckii Lac(+)”) and L. delbrueckii which cannot utilize lactose as an energy source, that is, which has no lactose utilization (hereinafter sometimes referred to as “L. delbrueckii Lac(−)”). Conventionally, it has been considered difficult to obtain a fermented food (especially fermented milk) having lactose as a main sugar source when using L. delbrueckii Lac(−). However, in the present invention, even with any of L. delbrueckii (even with L. delbrueckii Lac(−)), it is possible to stably obtain a fermented food in a short time by fermenting the milk preparation solution by combining with Streptococcus thermophilus carrying the following prtS gene.

In the present invention, the method for confirming whether or not L. delbrueckii has lactose utilization is not particularly limited. For example, even in the case of culture for 48 hours at a proper temperature and a proper pH in a medium containing lactose as a single sugar source, it can be confirmed by the fact that lactose in the medium cannot be catabolized into glucose and galactose, and that the pH of the medium is not changed by lactic acid.

The L. delbrueckii according to the present invention is not particularly limited and may be selected from those known as the above subspecies. Particularly from the viewpoint that a fermented food (especially fermented milk) having a balanced and mild flavor in all of acidity, umami, fermented aroma, and richness tends to be obtained,

-   -   preferable is Lactobacillus delbrueckii classified into any one         of clusters I, II, III, and V by MLSA classification based on         seven housekeeping genes consisting of fusA gene, gyrB gene,         hsp60 gene, ileS gene, pyrG gene, recA gene, and recG gene, and     -   more preferable is at least one selected from the group         consisting of L. delbrueckii satisfying all of the following         conditions (i) to (vii):     -   (i) carrying an fusA gene whose allele number is 2 or 14,     -   (ii) carrying a gyrB gene whose allele number is any one         selected from 3, 17, 18, and 25,     -   (iii) carrying an hsp60 gene whose allele number is any one         selected from 4, 18, 26, and 27,     -   (iv) carrying an ileS gene whose allele number is any one         selected from 16, 20, 21, 30, and 33,     -   (v) carrying a pyrG gene whose allele number is 22 or 23,     -   (vi) carrying an recA gene whose allele number is any one         selected from 2, 7, 24, and 28, and     -   (vii) carrying an recG gene whose allele number is 3, and L.         delbrueckii classified into the same cluster as that of the L.         delbrueckii satisfying all of the conditions (i) to (vii) by         MLSA classification based on seven housekeeping genes consisting         of fusA gene, gyrB gene, hsp60 gene, ileS gene, pyrG gene, recA         gene, and recG gene.

In the present invention, the “fusA gene” refers to a gene encoding elongation factor EF-2 (protein elongation factor ES-2), the “gyrB gene” refers to a gene encoding DNA gyrase subunit B, the “hsp60 gene” refers to a gene encoding heat-shock protein 60, the “ileS gene” refers to a gene encoding isoleucyl-tRNA synthase, the “pyrG gene” refers to a gene encoding CTP synthase, the “recA gene” refers to a gene encoding recombinase A, and the “recG gene” refers to a gene encoding ATP-dependent DNA helicase. All of these seven genes are housekeeping genes that are essential for the maintenance and growth of L. delbrueckii.

The MLSA classification based on these seven housekeeping genes is a classification based on multilocus sequence analysis (MLSA), which is a technique including obtaining a sequence (concatenated sequence) obtained by concatenating the base sequences of the above housekeeping genes for each strain, constructing a phylogenetic tree using the obtained concatenated sequence, and classifying strains into clusters based on the obtained phylogenetic tree. In the present invention, MLSA classification is carried out on 59 strains of L. delbrueckii with strain Nos. 1 to 59 presented in Tables 1 and 2 below (more preferably 61 strains with strain Nos. 1 to 61 presented in Tables 1 and 2 below), or, when the target strain is not included in the 59 strains (or the 61 strains), MLSA classification is carried out on 60 strains (or 62 strains) including the 59 strains (or the 61 strains) plus 1 strain being the target. In this way, the cluster of each strain is determined.

In Tables 1 and 2 below, the strains with strain Nos. 1 to 41 are L. delbrueckii described in Tanigawa et al., Microbiology, 2011, 157, pp. 727-738 (hereinafter referred to as “Tanigawa et al.”), and the strain with strain No. 42 is L. delbrueckii described in Adimpong D. B. et al., International Journal of Systematic and Evolutionally Microbiology, 2013, 63, pp. 3720-3726 (hereinafter referred to as “Adimpong D. B. et al.”). In addition, the strains with strain Nos. 43 to 61 are 19 strains of L. delbrueckii among several species of lactic acid bacteria derived from raw milk, yogurt, Japanese traditional pickles “sunki”, and the like.

The base sequence of each housekeeping gene was obtained from GeneBank/EMBL/DDBJ access numbers described in Tanigawa et al. for strain Nos. 1 to 41, and obtained from GeneBank/EMBL/DDBJ access number ALPY0000000 described in Adimpong D. B. et al. for strain No. 42. In addition, the base sequences for strain Nos. 43 to 61 were obtained as follows. In the method described in the Examples to be described later, the primers described later, prepared from the highly conserved sequences of the housekeeping genes, were used to obtain products by the PCR method, which were used to determine the base sequence of each gene. Among the obtained base sequences, a list of the sequence identification numbers of the base sequences newly obtained this time (base sequences not described in Tanigawa et al.; base sequences newly assigned the following allele numbers this time) is presented in Table 3 below together with allele numbers described later. The base sequences of the housekeeping genes of L. delbrueckii except for the 61 strains can be obtained by, for example, using the above-mentioned primers in the same manner as strain Nos. 43 to 61.

In the present invention, the concatenated sequence in each strain is obtained by concatenating the obtained base sequences for seven genes in the order of fusA gene, gyrB gene, hsp60 gene, ileS gene, pyrG gene, recA gene, and recG gene. As a method for constructing a phylogenetic tree using the above-mentioned concatenated sequence, a conventionally known method can be appropriately employed. For example, the phylogenetic tree can be constructed using software such as Genetyx v. 13 (manufactured by GENETYX CORPORATION), but it is preferable to use the unweighted pair group method with arithmetic mean (UPGMA). FIG. 1 illustrates a phylogenetic tree constructed by the unweighted pair group method with arithmetic mean with Genetyx v. 13 using the above concatenated sequence for L. delbrueckii with strain Nos. 1 to 61 described in Tables 1 and 2 below. In addition, Tables 1 and 2 below present clusters in each of which a strain is classified according to the phylogenetic tree obtained, together with allele numbers described later.

As presented in FIG. 1 and Tables 1 and 2 below, among clusters I classified by the phylogenetic tree obtained by the above method, subcluster I-B (cluster I-B) is a new cluster composed of novel L. delbrueckii without conventionally known L. delbrueckii (for example, L. delbrueckii with strain Nos. 1 to 42).

Particularly from the viewpoint that a fermented food (especially fermented milk) having a balanced and mild flavor in all of acidity, umami, fermented aroma, and richness tends to be obtained in combination with Streptococcus thermophilus carrying the following prtS gene, such L. delbrueckii is

-   -   preferably L. delbrueckii classified into any one of clusters I,         II, III, and V by the above MLSA classification,     -   more preferably at least one selected from the group consisting         of L. delbrueckii satisfying all of the following conditions (i)         to (vii):     -   (i) carrying an fusA gene whose allele number is 2 or 14,     -   (ii) carrying a gyrB gene whose allele number is any one         selected from 3, 17, 18, and 25,     -   (iii) carrying an hsp60 gene whose allele number is any one         selected from 4, 18, 26, and 27,     -   (iv) carrying an ileS gene whose allele number is any one         selected from 16, 20, 21, 30, and 33,     -   (v) carrying a pyrG gene whose allele number is 22 or 23,     -   (vi) carrying an recA gene whose allele number is any one         selected from 2, 7, 24, and 28, and     -   (vii) carrying an recG gene whose allele number is 3, and L.         delbrueckii classified into the same cluster (cluster I in FIG.         1 ) as that of the L. delbrueckii satisfying all of the         conditions (i) to (vii) by the above MLSA classification, and     -   further preferably at least one selected from the group         consisting of the L. delbrueckii satisfying all of the         conditions (i) to (vii) and the L. delbrueckii classified into         the same subcluster (subcluster I-B in FIG. 1) as that of the L.         delbrueckii satisfying all of the conditions (i) to (vii).

In the present invention, hereinafter, the “L. delbrueckii classified into anyone of clusters I, II, III, and V (hereinafter sometimes referred to as ‘L. delbrueckii (I, II, III, V)’)” refers to, when the cluster into which L. delbrueckii satisfying all of the conditions (i) to (vii) is classified is defined as cluster I, any one of L. delbrueckii classified into clusters II, III, and V excluding cluster IV, and L. delbrueckii classified into cluster I among the four clusters (clusters II to V) classified in order by the unweighted pair group method with arithmetic mean. Clusters I to V may be further divided into subclusters (for example, in FIG. 1 , cluster I is divided into subclusters I-A to I-G, and cluster III is divided into subclusters III-A and III-B).

Among these, particularly from the viewpoint that a fermented food (especially fermented milk) having a balanced and mild flavor in all of acidity, umami, fermented aroma, and richness tends to be obtained, the L. delbrueckii according to the present invention is preferably L. delbrueckii classified into cluster I, and particularly preferably L. delbrueckii classified into subcluster I-B (hereinafter sometimes referred to as “L. delbrueckii (I-B)”).

FIG. 2A to FIG. 8E illustrate a list of allele numbers of seven housekeeping genes consisting of fusA gene, gyrB gene, hsp60 gene, ileS gene, pyrG gene, recA gene, and recG gene according to the present invention, base sequences thereof, and polymorphic sites thereof (FIGS. 2A to 2C: fusA gene (the number following the hyphen (-) after “fusA” represents the allele number, and the same applies to the following), FIGS. 3A to 3D: gyrB gene, FIGS. 4A to 4F: hsp60 gene, FIGS. 5A to 5E: ileS gene, FIGS. 6A to 6D: pyrG gene, FIGS. 7A to 7E: recA gene, and FIGS. 8A to 8E: recG gene). FIG. 2A to FIG. 8E each present the entire base sequence with allele number 1 (the base sequence described in Tanigawa et al.), and present only different bases for the genes represented by the subsequent allele numbers. The allele number of each gene of the strains with strain Nos. 1 to 41 described in Tanigawa et al. basically represents the allele number described in Tanigawa et al. Note that, in Tanigawa et al., allele numbers 13 and 14 of the hsp60 gene and allele numbers 16 and 17 of the recG gene are assigned different numbers. Since they are the same single sequence as illustrated in FIGS. 4A to 4F and FIGS. 8A to 8E, these are written as allele numbers 13/14 and 16/17 in the present invention, as presented in Tables 1 and 2. For the allele number of each gene of the strains with strain Nos. 42 to 61, the base sequence of each housekeeping gene obtained by the method described in the above MLSA classification was compared with the above base sequences with strain Nos. 1 to 41 for each gene, and the base sequences that did not match any of the base sequences with strain Nos. 1 to 41 were assigned new allele numbers. Tables 1 and 2 below present the allele number of each gene of the above 61 strains. Tables 1 and 2 also present the cluster into which each strain is classified by the phylogenetic tree obtained by the above MLSA classification. In addition, Table 3 below presents a list of the sequence identification numbers and allele numbers of the base sequences of the housekeeping genes to which allele numbers were newly assigned this time (base sequences newly obtained this time; base sequences not described in Tanigawa et al.).

TABLE 1 Lactobacillus delbrueckii Allele Number Strain No. Strain Name Cluster fusA gyrB hsp60 ileS pyrG recA recG 1 YIT0080^(T) II 1 1 1 1 1 1 1 (=JCM 1012^(T)) 2 YIT11851 I-A 2 4 1 19 6 7 22 3 YIT12133 I-A 7 4 10 3 6 2 11 4 YIT12134 I-A 8 4 4 3 6 2 12 5 YIT0086^(T) I-C 2 3 3 2 18 17 3 (=JCM 1248^(T)) 6 YIT0020 I-C 3 3 3 3 3 2 3 7 YIT0058 I-C 2 3 3 3 3 2 3 8 YIT0059 I-C 3 3 3 3 3 2 3 9 YIT0087 I-D 2 4 4 4 4 3 4 10 YIT0167 I-D 2 4 4 4 4 4 5 11 YIT11852 I-C 3 3 3 3 3 2 3 12 YIT11853 I-C 3 3 3 3 3 2 3 13 YIT11854 I-C 3 3 3 3 3 4 3 14 YIT0181^(T) IV 4 5 5 5 5 5 6 15 YIT0044 IV 4 6 6 6 6 6 7 16 YIT0067 IV 4 8 5 8 8 6 6 17 YIT0459 IV 4 6 8 9 6 6 9 18 YIT0460 IV 4 6 6 6 6 6 7 19 YIT0463 IV 4 8 5 5 6 6 6 20 YIT0478 IV 4 6 6 10 6 6 7 21 YIT11751^(T) V 6 9 9 11 9 8 10 (=JCM 15610^(T)) 22 YIT12135 V 6 11 12 13 11 10 10 23 YIT12136 V 6 12 9 13 12 10 14 24 YIT12137 V 6 13 9 13 12 10 14 25 YIT0057 I-C 2 2 2 2 2 2 2 26 YIT0373 III-B 5 7 7 7 7 7 8 27 YIT11220 III-A 9 10 11 12 10 9 13 28 YIT11221^(T) III-A 9 10 11 17 10 9 13 (=JCM 17838^(T)) 29 YIT11435 I-E 2 4 3 14 13 7 15 30 YIT11451 I-E 2 4 3 14 13 7 15 31 YIT11466 III-A 9 10 11 17 17 9 13 32 YIT11468 I-E 2 4 3 14 13 7 15 33 YIT11501 I-F 2 3 15 16 6 11 18 34 YIT11502 I-E 10 4 13/14 15 14 7 16/17 35 YIT11639 I-E 10 4 13/14 15 14 7 16/17 36 YIT11673 III-A 9 10 11 17 10 9 13 37 YIT11291 I-E 2 16 3 14 6 16 21 38 YIT11293 I-E 2 16 3 14 6 15 20 39 YIT11294 I-E 11 14 16 14 15 12 8 40 YIT11300 I-E 12 4 3 18 6 13 15 41 YIT11403 I-G 12 15 17 14 16 14 19 42 DSM26046^(T) I-E 18 27 3 14 6 15 40 (=JCM 30917^(T))

TABLE 2 Lactobacillus delbrueckii Allele Number Strain No. Strain Name Cluster fusA gyrB hsp60 ileS pyrG recA recG 43 SQ-1 I-B 2 3 4 16 22 2 3 44 SQ-2 I-B 2 3 4 21 22 7 3 45 SQ-3 I-B 2 3 4 21 23 7 3 46 NITE ABP-02874 I-B 2 3 18 21 22 2 3 (SQ-4) 47 MEP1900404 I-B 2 3 18 21 22 7 3 (SQ-5) 48 SQ-6 I-B 2 3 18 21 23 2 3 49 MEP1900403 I-B 2 3 18 30 22 7 3 (SQ-7) 50 SQ-8 I-B 2 18 4 16 22 7 3 51 SQ-9 I-B 2 18 4 16 23 7 3 52 SQ-10 I-B 2 25 18 21 22 28 3 53 SQ-11 I-B 14 17 18 20 22 7 3 54 SQ-12 I-B 14 17 18 21 22 7 3 55 SQ-13 I-B 14 17 18 21 22 24 3 56 SQ-14 I-B 14 17 18 33 22 7 3 57 MEP1900401 I-A 13 4 4 3 6 11 16/17 (SQ-15) 58 MEP1900407 I-F 17 18 19 16 6 18 18 (SQ-16) 59 SQ-17 I-E 13 26 3 14 6 15 38 60 SQ-18 I-B 14 17 26 21 22 7 3 61 SQ-19 I-B 2 3 27 21 22 7 3

TABLE 3 Gene Allele Number SEQ ID NO fusA 13 1 14 2 17 3 18 4 gyrB 17 5 18 6 25 7 26 8 27 9 hsp60 18 10 19 11 26 42 27 12 ileS 18 13 19 14 20 15 21 16 30 17 33 18 pyrG 22 19 23 20 recA 18 21 24 22 28 23 recG 38 24 40 25

For L. delbrueckii except for the above 61 strains, for example, the base sequence of each housekeeping gene obtained by the method described in the above MLSA classification is compared with the above base sequences and allele numbers described in above FIG. 2A to FIG. 8E for each gene. Thereby, it is possible to obtain each allele number.

A preferable example of the L. delbrueckii satisfying all of the conditions (i) to (vii) is L. delbrueckii in which the allele numbers of the housekeeping genes carried are, in the order of fusA gene, gyrB gene, hsp60 gene, ileS gene, pyrG gene, recA gene, and recG gene,

-   -   2, 3, 4, 16, 22, 2, and 3;     -   2, 3, 4, 21, 22, 7, and 3;     -   2, 3, 4, 21, 23, 7, and 3;     -   2, 3, 18, 21, 22, 2, and 3;     -   2, 3, 18, 21, 22, 7, and 3;     -   2, 3, 18, 21, 23, 2, and 3;     -   2, 3, 18, 30, 22, 7, and 3;     -   2, 18, 4, 16, 22, 7, and 3;     -   2, 18, 4, 16, 23, 7, and 3;     -   2, 25, 18, 21, 22, 28, and 3;     -   14, 17, 18, 20, 22, 7, and 3;     -   14, 17, 18, 21, 22, 7, and 3;     -   14, 17, 18, 21, 22, 24, and 3;     -   14, 17, 18, 33, 22, 7, and 3;     -   14, 17, 26, 21, 22, 7, and 3; or     -   2, 3, 27, 21, 22, 7, and 3.

Among the L. delbrueckii, the L. delbrueckii according to the present invention is

-   -   more preferably at least one selected from the group consisting         of     -   L. delbrueckii specified by accession number NITE BP-02874         (receipt number NITE ABP-02874) (L. delbrueckii NITE ABP-02874:         strain No. 46 of Table 2) and     -   L. delbrueckii classified into the same cluster (that is,         cluster I) as that of the L. delbrueckii specified by accession         number NITE BP-02874 (receipt number NITE ABP-02874) by the         above MLSA classification,     -   further preferably at least one selected from the group         consisting of     -   the L. delbrueckii specified by accession number NITE BP-02874         and     -   L. delbrueckii classified into the same subcluster (that is,         subcluster I-B) as that of the L. delbrueckii specified by         accession number NITE BP-02874 (receipt number NITE ABP-02874)         by the above MLSA classification, and     -   particularly preferably     -   the L. delbrueckii specified by accession number NITE BP-02874.

The L. delbrueckii specified by accession number NITE BP-02874 has been deposited in (1) deposition institute: National Institute of Technology and Evaluation, NITE Patent Microorganisms Depositary (NPMD) (postal code: 292-0818, 2-5-8 Kazusa-Kamatari, Kisarazu-shi, Chiba Prefecture, Room 122) with (2) receipt date (original deposition date): Feb. 5, 2019, (3) accession number NITE BP-02874 (receipt number: NITE ABP-02874), and (4) identification label: Lactobacillus delbrueckii OLL204989. Note that the L. delbrueckii specified by accession number NITE BP-02874 may be a passaged strain of the same strain, or an artificial mutant strain, a natural mutant strain, or a genetically modified strain of the same strain or a passaged strain thereof as long as the effects of the present invention are not impaired (preferably, within a scope that satisfies the preferred conditions of the L. delbrueckii according to the present invention). The L. delbrueckii specified by accession number NITE BP-02874 is L. delbrueckii which is L. delbrueckii Lac(+) and L. delbrueckii (I-B), and in which the allele numbers of the housekeeping genes carried are, in the order of fusA gene, gyrB gene, hsp60 gene, ileS gene, pyrG gene, recA gene, and recG gene, 2, 3, 18, 21, 22, 2, and 3.

(Streptococcus Thermophilus)

The method for producing a fermented food of the present invention ferments the above-described milk preparation solution added with a combination of L. delbrueckii and Streptococcus thermophilus carrying a prtS gene.

The Streptococcus thermophilus according to the present invention (hereinafter sometimes referred to as “S. thermophilus”) is not particularly limited as long as it carries a prtS gene, and may be used alone or in combination of two or more. In the present invention, the “prtS gene” refers to a gene encoding a cell wall-bound serine protease that degradates casein.

In the present invention, whether or not S. thermophilus carries a prtS gene can be determined by, for example, whether or not a desired PCR product can be obtained by amplifying a part of the prtS gene using the following primers prepared from the highly conserved sequence of the prtS gene by the method described in the Examples below. By using S. thermophilus carrying the prtS gene (hereinafter sometimes referred to as “S. thermophilus prtS(+)”), even when combined with the L. delbrueckii Lac(−), the fermentation time can be significantly reduced as compared with the case of using S. thermophilus carrying no prtS gene (hereinafter sometimes referred to as “S. thermophilus prtS(−)”). Moreover, combination of the S. thermophilus prtS(+) and the L. delbrueckii classified into any one of clusters I, II, III, and V (more preferably cluster I, and further preferably subcluster I-B) by the above MLSA classification makes it possible to particularly obtain a fermented food having a balanced and mild flavor in all of acidity, umami, fermented aroma, and richness.

Such S. thermophilus prtS(+) is preferably S. thermophilus specified by accession number NITE BP-02875 (receipt number NITE ABP-02875). The S. thermophilus specified by accession number NITE BP-02875 is S. thermophilus prtS(+) derived from Japanese raw milk.

The S. thermophilus specified by accession number NITE BP-02875 has been deposited in (1) deposition institute: National Institute of Technology and Evaluation, NITE Patent Microorganisms Depositary (NPMD) (postal code: 292-0818, 2-5-8 Kazusa-Kamatari, Kisarazu-shi, Chiba Prefecture, Room 122) with (2) receipt date (original deposition date): Feb. 5, 2019, (3) accession number NITE BP-02875 (receipt number: NITE ABP-02875), and (4) identification label: Streptococcus thermophilus OLS4496. Note that the S. thermophilus specified by accession number NITE BP-02875 may be a passaged strain of the same strain, or an artificial mutant strain, a natural mutant strain, or a genetically modified strain of the same strain or a passaged strain thereof within a scope that satisfies the conditions according to the present invention.

(Fermentation)

As the fermentation step of fermenting above-described milk preparation solution added with the L. delbrueckii and the S. thermophilus prtS(+), a conventionally known method can be appropriately employed without particular limitation. The method is, for example, a method for fermenting the milk preparation solution inoculated with the L. delbrueckii and the S. thermophilus prtS(+) as fermentation starters.

The L. delbrueckii and the S. thermophilus prtS(+) may be, respectively, in the form of a lactic acid bacteria-containing composition and/or a processed product of the lactic acid bacterium, which contains both of these lactic acid bacteria or independently contains them. In the present invention, the lactic acid bacteria-containing composition includes a culture supernatant after the end of the culture of the lactic acid bacteria (L. delbrueckii and/or S. thermophilus prtS(+)) and a culture which is a medium component and the like; and a concentrate, a dilution, a dried product, a frozen product, and the like of the culture, or may include one of these alone or a combination of two or more thereof. In addition, in the present invention, the lactic acid bacteria-containing composition and/or the processed product of the lactic acid bacterium (lactic acid bacteria) include a crushed product and heat-treated product of the lactic acid bacterium (lactic acid bacteria) or the culture, their concentrate, dilution, dried product, frozen product, and the like, may include one of these alone or a combination of two or more thereof. Among these, the L. delbrueckii and the S. thermophilus prtS(+) are preferably added to the milk preparation solution in the form of the lactic acid bacteria-containing composition, and more preferably in the form of a culture or a concentrate of the culture.

The amount of the fermentation starters added can be appropriately set according to the addition amount employed in a conventionally known method for producing a fermented food, and is, for example, preferably 1×10⁷ to 5×10⁹ CFU/mL, and more preferably 1×10⁸ to 2×10⁹ CFU/mL relative to the volume of the milk preparation solution in terms of the number of lactic acid bacteria (total number of bacteria of L. delbrueckii and S. thermophilus prtS(+)).

In addition, the ratio of the number of bacteria between L. delbrueckii and S. thermophilus prtS(+) (L. delbrueckii:S. thermophilus prtS(+)) added to the milk preparation solution is preferably 1:0.1 to 1:100, and more preferably 1:1 to 1:10.

The method for inoculating the fermentation starters is not particularly limited, and a method commonly used in a method for producing a fermented food can be appropriately used. The L. delbrueckii and the S. thermophilus prtS(+) may be inoculated simultaneously with each other alone, or may be inoculated simultaneously after mixing them (for example, in the form of a lactic acid bacteria-containing composition containing both of them). The fermentation conditions can be appropriately selected without particular limitation according to the growth conditions of L. delbrueckii and S. thermophilus prtS(+) to be added, the amount of the milk preparation solution, and the like. For example, it is preferable to be static or stir (preferably be static) the milk preparation solution normally for 3 to 24 hours, more preferably 3 to 8 hours, and further preferably 4 to 6 hours at a temperature of 35 to 45° C. and more preferably a temperature of 38 to 43° C. under aerobic or anaerobic conditions until the pH of the milk preparation solution added with the L. delbrueckii and S. thermophilus prtS (+) reaches a value of 4.8 or less and more preferably a value between 4.0 and 4.6. The present invention makes it possible to significantly shorten the time required for fermentation even when L. delbrueckii Lac(−) is used. In addition, as the anaerobic condition, for example, fermentation under a nitrogen aerated condition can be employed.

The above fermentation makes it possible to obtain the fermented food of the present invention. The fermented product after the fermentation step can be used as the fermented food of the present invention as it is, or by concentrating, diluting, drying, or freezing as necessary. In addition, the fermented food of the present invention may be obtained by crushing or heating the lactic acid bacteria in the fermented product, or by concentrating, diluting, drying, or freezing them as necessary.

<Fermented Food>

The fermented food of the present invention contains the L. delbrueckii and the S. thermophilus prtS(+), and can be obtained by the above-described method for producing a fermented food of the present invention.

The fermented food of the present invention is not particularly limited, and may be, for example, any of the fermented foods defined in the Ministerial Ordinance on Milk and Milk products Concerning Compositional Standards, etc. by the Ministry of Health, Labor and Welfare of Japan: fermented milk satisfying the specifications of “fermented milk” (more specifically, the milk solids-not-fat content is 8.0% or more, and the lactic acid bacteria count or yeast count (preferably the lactic acid bacteria count (more preferably the total count of L. delbrueckii and S. thermophilus prtS(+), the same applies hereinafter)) is 10,000,000/mL or more), those satisfying the specifications of “milk product/lactic acid bacteria drink” (more specifically, the milk solids-not-fat content is 3.0% or more, and the lactic acid bacteria count or yeast count (preferably the lactic acid bacteria count) is 10,000,000/mL or more), and those satisfying the specifications of “lactic acid bacteria drink” (more specifically, the milk solids-not-fat content is less than 3.0%, and the lactic acid bacteria count or yeast count (preferably the lactic acid bacteria count) is 1,000,000/mL or more). Note that the milk solids-not-fat indicates the remaining components obtained by subtracting the fat content from the whole milk solid content (mainly, protein, lactose, and minerals), and the lactic acid bacteria count and yeast count are measured by a test method specified by the Ministerial Ordinance before sterilization.

The fermented food of the present invention may be a fermented product after the fermentation step, may be obtained by sterilizing the fermented product, or may be obtained by concentrating, diluting, drying, or freezing them. For example, the fermented milk may be a sterilized treatment product of the above-described fermented milk, milk product/lactic acid bacteria drink, or lactic acid bacteria drink. In this case, the lactic acid bacteria count (preferably the total count of L. delbrueckii and S. thermophilus prtS (+)) is in terms of live bacteria count. The lactic acid bacteria contained in the fermented food of the present invention include not only live bacteria but also dead bacteria, and include crushed products and heat-treated products of lactic acid bacteria, their concentrates, dilutions, dried products, and frozen products. Here, the lactic acid bacteria contained in the fermented food of the present invention preferably contain at least live bacteria.

The fermented food of the present invention may contain, as lactic acid bacteria, other lactic acid bacteria except for the L. delbrueckii and S. thermophilus prtS(+) according to the present invention, and may further contain yeast, as long as the effects of the present invention are not impaired. Examples of these other lactic acid bacteria and yeasts include lactic acid bacteria and yeasts conventionally known to be contained in fermented foods (such as fermented milk).

In addition, the fermented food of the present invention may further contain various components that can be contained in foods and drinks. Such components are not particularly limited, and examples thereof include water, saccharides, sugar alcohols, minerals, vitamins, proteins, peptides, amino acids, organic acids, pH adjusters, starch and processed starch, dietary fibers, fruits and vegetables and processed products thereof, animal and plant crude drug extracts, naturally derived polymers (such as collagen, hyaluronic acid, and chondroitin), oils and fats, thickeners, emulsifiers, solvents, surfactants, gelling agents, stabilizers, buffers, suspending agents, thickening agents, excipients, disintegrants, binders, flow agents, preservatives, coloring agents, flavors, corrigents, and sweeteners. One of these may be contained alone or in combination of two or more.

Such a fermented food is preferably fermented milk, and the fermented milk is preferably yogurt, cheese, fermented cream, fermented butter, and the like, and particularly preferably yogurt. Specific examples of the yogurt include set type yogurt (solid fermented milk) such as plain yogurt, soft type yogurt (pasty fermented milk), and drink type yogurt (liquid fermented milk). Frozen yogurt using these as materials may be employed. In addition, the fermented food of the present invention can also be used as a material for fermented foods such as cheese, fermented cream, fermented butter, and kefir.

EXAMPLES

Hereinafter, the present invention is described more specifically based on Examples and Comparative Examples, but the present invention is not limited to the following Examples. In the following Examples and Comparative Examples, the following methods were used to perform the confirmation of the presence or absence of the prtS gene carried by S. thermophilus, the confirmation of the presence or absence of lactose utilization of L. delbrueckii, and the cluster classification (MLSA classification) of L. delbrueckii.

<Detection of prtS Gene in S. Thermophilus>

The prtS gene sequences of five strains of S. thermophilus with known genomic sequences were obtained from the NCBI databases, and the following primers were prepared from highly conserved sequences (sequence identification number in the parentheses indicates the base sequence of each primer):

-   -   prtS gene forward primer (SEQ ID NO: 26)     -   prtS gene reverse primer (SEQ ID NO: 27).

In addition, InstaGene Matrix (manufactured by BioRad) was used to extract genomic DNA from the M17 culture of each strain. The extracted genomic DNA (template) in an amount of 0.5 μL, 1 μL of each prepared primer (5 μM), 0.1 μL of Phusion high fidelity DNA polymerase, 2 μL of 5×HF buffer, 0.8 μL of 2.5 mM dNTP, and 4.6 μL of ultrapure water were mixed (total of 10 μL), followed by PCR under the conditions presented in Table 4 below. The obtained PCR products were subjected to agarose gel electrophoresis, and the strain with a band observed at the position of 684 bp was determined as S. thermophilus carrying a prtS gene (S. thermophilus prtS(+)) and the strain without a band observed was determined as S. thermophilus carrying no prtS gene (S. thermophilus prtS(−)).

TABLE 4 PCR Condition 98° C. 30 s    98° C.  5 s 30 cycles 63° C. 20 s 72° C. 20 s 72° C. 5 min  4° C. ∞ <Assessment of a Lactose Utilization by L. Delbrueckii>

First, a modified MRS medium (purple) was prepared having no single sugar source (added with water) or glucose or lactose (for positive control: glucose; for negative control: water), and further added with a pH indicator (bromocresol purple: BCP). Table 5 below presents the composition of the modified MRS medium. Next, each of the L. delbrueckii strains to be evaluated was subjected to activation culture twice in an MRS Broth (37° C., anaerobic, cultured for 18 hours), and then the culture was centrifuged (2000×g, 4° C., 20 minutes), and the supernatant was discarded to collect the bacterial cells. Physiological saline was added to the collected bacterial cells to prepare a two-fold concentrated bacterial suspension. This bacterial suspension was added to a modified MRS medium at the ratio of 3.25% (mL/100 mL), and the mixture was incubated statically at 37° C. for 48 hours. After the end of the culture, the color of the culture was observed, and the utilization of lactose was confirmed according to the following criteria:

-   -   without lactose utilization (Lac(−)): the color of the culture         having lactose as the single sugar source does not change even         after the end of the culture, and it remains purple, which is         the same as the color of the culture having no single sugar         source (water) after the end of the culture, and is different         from the color of the culture solution having glucose as the         single sugar source after the end of the culture, and     -   with lactose utilization (Lac(+)): the color of the culture         having lactose as the single sugar source after the end of the         culture has changed to yellow, and is the same as the color of         the culture having glucose as the single sugar source after the         end of the culture.

TABLE 5 For 1 L Sugar 10 g Proteose Peptone No. 3 10 g Yeast Extract 5 g Polysorbate 80 1 ml Triammonium Citrate 2 g Sodium Acetate 5 g Magnesium Sulfate Heptahydrate 0.1 g Manganese Sulfate Pentahydrate 0.05 g K₂HPO₄ 2 g Bromocresol Purple 0.17 g <Cluster Classification (MLSA Classification) of L. Delbrueckii>

(1) MLSA classification was performed on 59 strains of L. delbrueckii. Specifically, first, the same method as that described in Tanigawa et al. was used for 17 strains of L. delbrueckii presented in strain Nos. 43 to 59 in Table 2 above, and the following primers (sequence identification number in the parentheses indicates the base sequence of each primer), prepared from highly conserved sequences of fusA gene, gyrB gene, hsp60 gene, ileS gene, pyrG gene, recA gene, and recG gene, were used to amplify the sequences of the genes by the PCR method:

-   -   fusA gene: forward primer (SEQ ID NO: 28) and reverse primer         (SEQ ID NO: 29),     -   gyrB gene: forward primer (SEQ ID NO: 30) and reverse primer         (SEQ ID NO: 31),     -   hsp60 gene: forward primer (SEQ ID NO: 32) and reverse primer         (SEQ ID NO: 33),     -   ileS gene: forward primer (SEQ ID NO: 34) and reverse primer         (SEQ ID NO: 35),     -   pyrG gene: forward primer (SEQ ID NO: 36) and reverse primer         (SEQ ID NO: 37),     -   recA gene: forward primer (SEQ ID NO: 38) and reverse primer         (SEQ ID NO: 39), and     -   recG gene: forward primer (SEQ ID NO: 40) and reverse primer         (SEQ ID NO: 41). Next, the base sequences of the obtained PCR         products were determined, and for each strain, the base         sequences of seven genes were concatenated in the order of fusA         gene, gyrB gene, hsp60 gene, ileS gene, pyrG gene, recA gene,         and recG gene to obtain a concatenated sequence.

In addition, the base sequences of fusA gene, gyrB gene, hsp60 gene, ileS gene, pyrG gene, recA gene, and recG gene of 41 strains described in Tanigawa et al. (strains with strain Nos. 1 to 41 of Table 1) and 1 strain described in Adimpong D. B. et al. (strain with strain No. 42 of Table 1) were obtained from the corresponding GeneBank/EMBL/DDBJ access numbers described in the documents and concatenated to each other in the same manner as above to obtain concatenated sequences.

Next, a phylogenetic tree was constructed for all of the 59 strains described above by the unweighted pair group method with arithmetic mean (UPGMA) using Genetyx v. 13 (manufactured by GENETYX CORPORATION) based on the obtained concatenated sequences.

In addition, also for the 2 strains of L. delbrueckii with strain Nos. 60 and 61 presented in Table 2 above, the above primers were used to amplify the sequences of the genes by the PCR method, and a concatenated sequence was obtained for each strain. Subsequently, 2 strains of L. delbrueckii with strain Nos. 60 and 61 were added to the above 59 strains, and a phylogenetic tree was constructed for all 61 strains in the same manner as described above. In the 59 strains with strain Nos. 1 to 59, the clusters classified by the phylogenetic tree obtained by the 59 strains and the clusters classified by the phylogenetic tree obtained by the 61 strains were identical to each other, including the subclusters. FIG. 1 above illustrates a phylogenetic tree obtained from all the 61 strains. In addition, the clusters classified by the phylogenetic trees are presented in Tables 1 and 2.

(2) For L. delbrueckii described in Table 6 below except for the 61 strains, first, in the same manner as in the strains with strain Nos. 43 to 59, the base sequence and concatenated sequence for the seven genes were obtained. Next, a phylogenetic tree was constructed in the same manner as in (1) above from the concatenated sequences of all of the 59 strains described above and the obtained concatenated sequence of one target strain (concatenated sequences of all 60 strains). In the obtained phylogenetic tree, the cluster into which the target strain was classified was defined as the cluster of that strain.

<Measurement of Fermentation Time>

The time required for fermentation was measured using the strains of L. delbrueckii which are presented in Table 6 below and for which the presence or absence of lactose utilization and clusters were confirmed by the above method in combination with the strain of S. thermophilus carrying no prtS gene (S. thermophilus 1131, isolated from Meiji Bulgaria Yogurt LB81 (manufactured by Meiji Co., Ltd.)) or the strain of S. thermophilus carrying a prtS gene (S. thermophilus NITE ABP-02875, derived from Japanese raw milk, S. thermophilus specified by accession number NITE BP-02875 (receipt number NITE ABP-02875)), both of which had been confirmed for the presence or absence of prtS gene carried, in the combinations of Table 6 below (Y1 to Y20, Y6K(−): Comparative Examples; Y21 to Y40, Y6K(+): Examples; S1 and S2: Reference Examples). Table 6 also presents the presence or absence of the prtS gene carried by each S. thermophilus (prtS(+)/prtS(−)), the cluster of each L. delbrueckii by the MLSA classification (MLSA cluster), and lactose utilization (+: Lac(+)/−: Lac(−)).

TABLE 6 S. thermophilus prtS(+) Lactose NITE MLSA Utili- prtS(−) ABP- Cluster Strain Name zation 1131 02875 L. I-A MEP1900401 + Y1 Y21 delbrueckii MEP1900402 + Y2 Y22 I-B MEP1900403 + Y3 Y23 MEP1900404 + Y4 Y24 NITE ABP-02874 + Y6K(−) Y6K(+) I-C JCM 1248^(T) + Y5 Y25 MEP1900405 + Y6 Y26 I-E JCM 30917^(T) − Y7 Y27 MEP1900406 + Y8 Y28 I-F MEP1900407 + Y9 Y29 MEP1900408 + Y10 Y30 III-A JCM 17838^(T) − Y11 Y31 MEP1900409 − Y12 Y32 III-B MEP1900410 + Y13 Y33 MEP1900411 + Y14 Y34 II JCM 1012^(T) − Y15 Y35 IV 2038 + Y16 Y36 NITE BP-76 + Y17 Y37 MEP1900412 + Y18 Y38 V JCM 15610^(T) + Y19 Y39 MEP1900413 + Y20 Y40 - (S. thermophilus alone) S1 S2

The fermentation was as follows. Commercially available milk was added with each strain at 0.5% (0.5 mL/100 mL, the same applies to the following) or a combination thereof (L. delbrueckii:S. thermophilus=about 1:1 (bacterial count)) so that the amount of each strain was 0.5% (1% in total), followed by fermentation at 43° C. for 24 hours (aerobic, static culture) to obtain various types of fermented milk. The time from the addition of each strain until the pH reached 4.5 was measured and defined as the fermentation time required for fermentation. Table 7 below presents the results. Note that, if the pH did not reach 4.5 within 24 hours from the addition of strain, the fermentation was terminated with the fermentation time defined as 24 hours (1440 minutes).

TABLE 7 Combination Fermentation Combination Fermentation No. Time No. Time (prtS(−)) [min] (prtS(+)) [min] Y1 897 Y21 462 Y2 1440 Y22 498 Y3 432 Y23 348 Y4 417 Y24 348 Y6K(−) 396 Y6K(+) 327 Y5 1056 Y25 546 Y6 684 Y26 411 Y7 1440 Y27 468 Y8 768 Y28 429 Y9 375 Y29 309 Y10 480 Y30 393 Y11 849 Y31 522 Y12 942 Y32 507 Y13 642 Y33 471 Y14 921 Y34 474 Y15 1440 Y35 573 Y16 711 Y36 435 Y17 528 Y37 384 Y18 540 Y38 468 Y19 567 Y39 438 Y20 465 Y40 369 Average 761 Average 437 SD 346 SD 73 S1 1440 S2 699

In addition, FIG. 9 illustrates fermentation times (average values, minutes) when using S. thermophilus prtS(−) (Y1 to Y20, Y6K(−) (n=21)) (prtS(−)) and when using S. thermophilus prtS(+) (Y21 to Y40, Y6K(+) (n=21)) (prtS(+)). A comparison between average fermentation time when using of prtS(−) and one when using of prtS(+) by t-test showed that p-value was less than 0.05. As illustrated in FIG. 9 , it was confirmed that use of S. thermophilus prtS(+) significantly shortened the fermentation time as compared with the case of using S. thermophilus prtS(−). In addition, as presented in Table 7, it was confirmed that use of S. thermophilus prtS(+) shortened the fermentation time even in the case of a single S. thermophilus strain.

FIG. 10 illustrates the fermentation times (average values, minutes) depending on the presence or absence of lactose utilization of L. delbrueckii for each of these S. thermophilus. Specifically, the figure provides the fermentation time for each of the cases of using S. thermophilus prtS(−) and L. delbrueckii Lac(+) (prtS(−)-Lac(+): Y1 to Y6, Y8 to Y10, Y13, Y14, Y16 to Y20, Y6K(−), n=17), using S. thermophilus prtS(−) and L. delbrueckii Lac(−) (prtS(−)-Lac(−): Y7, Y11, Y12, Y15, n=4), using S. thermophilus prtS(+) and L. delbrueckii Lac(+) (prtS(+)-Lac(+): Y21 to Y26, Y28 to Y30, Y33, Y34, Y36 to Y40, Y6K(+), n=17), and using S. thermophilus prtS(+) and L. delbrueckii Lac(−) (prtS(+)-Lac(−): Y27, Y31, Y32, Y35, n=4). A comparison between average fermentation time when using of Lac(−) and one when using of Lac(+) by t-test showed that p-value was less than 0.05. As illustrated in FIG. 10 , the fermentation time was longer in the case of using L. delbrueckii Lac(−) than in the case of using L. delbrueckii Lac(+) regardless of which S. thermophilus was used. However, in the case of using S. thermophilus prtS(+), it was confirmed that the fermentation time was significantly shortened even in combination with such L. delbrueckii Lac(−).

<Analysis of Water-Soluble Component (in the Fermented Milk)>

In the above fermentation time measurement, the water-soluble components in the fermented milk obtained using various combinations of strains were analyzed by capillary electrophoresis-time-of-flight mass spectrometer (CE-TOF MS) according to the following method (Human Metabolome Technologies Inc.).

(Pretreatment)

To 900 μL of a methanol solution prepared so that the concentration of the internal standard substance was 10 μM, 100 μL of fermented milk was added, followed by stirring. Chloroform in an amount of 1,000 μL and ultrapure water in an amount of 400 μL were added thereto, followed by stirring and centrifugation (2,300×g, 4° C., 5 minutes). After centrifugation, 400 μL of the aqueous layer was transferred to an ultrafiltration tube (Ultrafree MC PLHCC, HMT, centrifugal filter unit 5 kDa). This was centrifuged (9,100×g, 4° C., 120 minutes) and subjected to ultrafiltration treatment. The filtrate was dried and dissolved again in 50 μL of ultrapure water for measurement.

(Measurement)

The cationic mode and the anionic mode were measured under the conditions presented in Table 8 below.

TABLE 8 Cationic Metabolite (Cationic Mode) Apparatus Agilent CE-TOF MS system (manufactured by Agilent Technologies) Capillary: Fused silica capillary i.d. 50 μm × 80 cm Measurement Run buffer: Cation Buffer Solution (p/n: H3301-1001) Condition Rinse buffer: Cation Buffer Solution (p/n: H3301-1001) Sample injection: Pressure injection 50 mbar, 10 sec CE voltage: Positive, 27 kV MS ionization: ESI Positive MS capillary voltage: 4,000 V MS scan range: m/z 50-1,000 Sheath liquid: HMT Sheath Liquid (p/n: H3301-1020) Anionic Metabolite (Cationic Mode) Run buffer: Anion Buffer Solution (p/n: H3302-1021) Rinse buffer: Anion Buffer Solution (p/n: H3302-1021) Sample injection: Pressure injection 50 mbar, 25 sec CE voltage: Positive, 30 kV MS ionization: ESI Negative MS capillary voltage: 3,500 V MS scan range: m/z 50-1,000 Sheath liquid: HMT Sheath Liquid (p/n: H3301-1020) (Data Processing)

For the peaks detected by CE-TOF MS, automatic integration software MasterHands ver. 2.17.1.11 (developed by Keio University) was used to automatically extract peaks having a signal/noise (S/N) ratio of 3 or more, thereby obtaining a mass-to-charge ratio (m/z), a peak area value, and a migration time (MT). The obtained peak area value was converted into a relative area value using the following equation: relative area value=(area value of target peak)/(area value of internal standard substance×amount of sample).

In addition, since these datasets included adduct ions such as Na⁺ and K⁺ and fragment ions such as dehydration and deammoniation, these molecular weight-related ions were deleted. However, due to the presence of substance-specific adducts and fragments, it was impossible to closely examine all of them. For the peaks examined closely, the peaks of the samples were collated and aligned based on the values of m/z and MT.

(Search for Candidate Metabolites)

Based on the values of m/z and MT, the detected peaks were collated and searched against all substances registered in the HMT metabolite library and the known-unknown library (Human Metabolome Technologies Inc.). The tolerable error for the search was ±0.5 min for MT and ±10 ppm for m/z (mass error (ppm)=(measured value−theoretical value)×10⁶/measured value). Note that, in the case where the same candidate metabolite was assigned to more than one peak because the candidates could not be narrowed down, peak numbers were assigned for labeling.

(Quantification of Target Metabolic Compound)

The target metabolic compound was analyzed. For the calibration curve, the peak area corrected by the internal standard substance was used, and the concentration was calculated for each substance as one point calibration with 100 μM (internal standard substance: 200 μM).

(Results)

(1) Principal component analysis (PCA) was performed on the water-soluble components in the fermented milk obtained by various combinations described in Table 6, where the combination is that of various S. thermophilus with L. delbrueckii classified into subcluster I-B of cluster I (L. delbrueckii (I-B)) or L. delbrueckii classified into cluster IV (L. delbrueckii (IV)). FIG. 11 and FIG. 12 illustrate the relationship between the first principal component score (PC1) and the second principal component score (PC2) in various types of fermented milk, FIG. 13 illustrates the relationship between the first principal component score (PC1) and the third principal component score (PC3), and FIG. 14 illustrates the relationship between the second principal component score (PC2) and the third principal component score (PC3). As illustrated in FIG. 11 , the results were divided into two depending on the presence or absence of the prtS gene carried by S. thermophilus (prtS(+)/prtS(−)), suggesting that the presence or absence of the prtS gene had a significant effect on the water-soluble components in fermented milk. In addition, as illustrated in FIGS. 12 to 14 , the results were also divided by the cluster of L. delbrueckii. In particular, in the case of using S. thermophilus prtS(+), the results were divided into two between L. delbrueckii (I-B) and L. delbrueckii (IV), suggesting that the cluster of L. delbrueckii also had an effect on the water-soluble components in fermented milk.

(2) Principal component analysis (PCA) was performed on the water-soluble components in the fermented milk obtained by combinations (Y16, Y36, Y6K(−), Y6K(+)) of various S. thermophilus with L. delbrueckii NITE ABP-02874 being L. delbrueckii classified into subcluster I-B of cluster I (L. delbrueckii specified by accession number NITE BP-02874 (receipt number NITE ABP-02874)) or L. delbrueckii 2038 being L. delbrueckii classified into cluster IV (isolated from Meiji Bulgaria Yogurt LB81 (manufactured by Meiji Co., Ltd.)). FIG. 15 illustrates the relationship between the first principal component score (PC1) and the second principal component score (PC2) in various types of fermented milk. In addition, FIG. 16 illustrates a scatter diagram of the water-soluble components corresponding to FIG. 15 . As illustrated in FIGS. 15 and 16 , those with higher amount of amino acid, carbohydrate metabolism-related substance, and lactic acid were plotted in the positive direction of the PC1 axis, and those with higher amount of ornithine and polyamine were plotted in the positive direction of the PC2 axis.

<Analysis of Aroma Component (in the Fermented Milk)>

In the above fermentation time measurement, the aroma components in the fermented milk obtained using various combinations of strains were analyzed by the dynamic headspace gas chromatography/mass spectrometry (GC/MS) method in accordance with the following method.

(Analysis Method)

To a 20-mL vial, 5 g of fermented milk, 5 g of 1 mol/L-phosphate buffer (pH 6.98), and methyl isobutyl ketone as the internal standard were added, and the vial was sealed. A dynamic headspace collection apparatus (manufactured by Gerstel Inc.) was used to replace the headspace with 10 mL of nitrogen gas while keeping the vial at 25° C., and the aroma components in the nitrogen gas were collected to an adsorbent (TENAX-TA). Next, the adsorbent was subjected to thermal desorption under the conditions presented in Table 9 below, and introduced and analyzed by GC/MS. The mass spectra of the detected peaks were compared with the NIST mass spectrum library to qualify the detected compound. Moreover, the peaks were integrated using ions specific to each compound to obtain a detected amount. In addition, if necessary, the value obtained by dividing the peak area by the peak area of the internal standard was used as the detection amount.

TABLE 9 Thermal TDU (manufactured by Gerstel Inc.) Desorption Apparatus Thermal 25° C. (0.5 min) → 720° C./min → 230° C. (5 min) Desorption Temperature Cryofocus −10° C. (0.5 min) → 720° C./min → 240° C. (10 min) GCMS 6890GC/5975MS (manufactured by Apparatus Agilent Technologies) Column DB-WAX UI inner diameter 0.25 mm × film thickness 0.25 μm × length 30 m Oven 40° C. (2.5 min) → 5° C./min → 80° C. → 10° C./ min → 120° C. → 20° C./min → 240° C. (5 min) Helium Gas 1 mL/min Scan Range m/z 33 to 300 (Results)

(1) Principal component analysis (PCA) was performed on the aroma components in the fermented milk obtained by various combinations described in Table 6, where the combination is that of various S. thermophilus with L. delbrueckii (I-B) or L. delbrueckii (IV). FIG. 17 illustrates the relationship between the first principal component score (PC1) and the second principal component score (PC2) in various types of fermented milk. As illustrated in FIG. 17 , also for the aroma components, the results were divided depending on the presence or absence of the prtS gene carried by S. thermophilus and on the difference in L. delbrueckii cluster, suggesting that S. thermophilus prtS(+) had a further effect on the aroma components in the fermented milk particularly in the case of using L. delbrueckii (I-B).

(2) Principal component analysis (PCA) was performed on the aroma components in the fermented milk obtained by combinations (Y16, Y36, Y6K(−), Y6K(+)) of various S. thermophilus with L. delbrueckii NITE ABP-02874 being L. delbrueckii classified into subcluster I-B of cluster I or L. delbrueckii 2038 being L. delbrueckii classified into cluster IV. FIG. 18 illustrates the relationship between the first principal component score (PC1) and the second principal component score (PC2) in various types of fermented milk. In addition, FIG. 19 illustrates a scatter diagram of the aroma components corresponding to FIG. 18 . As illustrated in FIGS. 18 and 19 , those with higher amount of acetaldehyde were plotted in the area with negative PC1 axis and negative PC2 axis, where the fermented milk using the combination (Y16) of S. thermophilus prtS(−) and L. delbrueckii (IV) (L. delbrueckii 2038) was plotted. Meanwhile, those with lower amount of acetaldehyde and higher amount of benzaldehyde (also known as an almond-like or apricot-like aroma), methylbutanal (also known as a burnt odor, malt odor or cocoa-like aroma), and DMS (also known as nori seaweed-like aroma or cow-like aroma) were plotted in the area with positive PC1 axis and positive PC2 axis, where the fermented milk using the combination (Y6K(+)) of S. thermophilus prtS(+) and L. delbrueckii (I-B) (L. delbrueckii NITE ABP-02874) was plotted.

<Sensory Evaluation>

In the above fermentation time measurement, the sensory evaluation of the fermented milk obtained using various combinations of strains presented in Table 6 was performed according to the following method.

(Evaluation Method)

The fermented milk after fermentation was immediately cooled on ice, and sensory evaluation was performed by a total of six trained panelists. The evaluation items were the following 11 items: acidity, sweetness, bitterness, umami, astringency/unpleasantness, milkiness, yogurtness, cheesiness (cream cheese-like), fattiness (butter-like or the like), milky full-bodiness/richness (cream-like or the like), and refreshing aftertaste, each of which was evaluated according to a 7-point scale of 1 to 7. The value obtained by averaging the evaluation results for six panelists was used as the sensory evaluation value.

(Results)

(1) FIG. 20 presents a diagram illustrating the relationship between the first principal component score (PC1) and the second principal component score (PC2) in various types of fermented milk obtained by performing principal component analysis (PCA), relating to the sensory evaluation value of the fermented milk obtained by various combinations of S. thermophilus prtS(−) and various L. delbrueckii described in Table 6. In addition, FIG. 21 illustrates a scatter diagram of the sensory evaluation items corresponding to FIG. 20 . Moreover, FIG. 22 presents a diagram illustrating the relationship between the first principal component score (PC1) and the second principal component score (PC2) in various types of fermented milk obtained by performing principal component analysis (PCA), relating to the sensory evaluation value of the fermented milk obtained by various combinations of S. thermophilus prtS(+) and various L. delbrueckii, and FIG. 23 illustrates a scatter diagram of the sensory evaluation items corresponding to FIG. 22 .

As illustrated in FIG. 20 and FIG. 21 , the results were such that, in fermented milk using S. thermophilus prtS(−) in combination with L. delbrueckii (IV), many of the panelists felt yogurtness and acidity, while in the fermented milk using S. thermophilus prtS(−) in combination with L. delbrueckii (I-B), many of the panelists felt umami and astringency. On the other hand, as illustrated in FIG. 22 and FIG. 23 , in fermented milk using S. thermophilus prtS(+) in combination with L. delbrueckii (IV), many of the panelists felt acidity and astringency, while in fermented milk using S. thermophilus prtS(+) in combination with L. delbrueckii classified into clusters I, II, III, and V (particularly L. delbrueckii (I-B)), many of the panelists made an evaluation as having a well-balanced, mild flavor as a whole with a milky full-bodiness/richness and milkiness. As illustrated in FIG. 20 to FIG. 23 , the results were divided depending on the L. delbrueckii cluster. In particularly, in the case of using S. thermophilus prtS (+), the results were largely divided into two between L. delbrueckii classified into clusters I, II, III, and V and L. delbrueckii (IV) classified into cluster IV. It was confirmed that the combination of S. thermophilus prtS(+) with L. delbrueckii classified into clusters I, II, III, and V (more preferably L. delbrueckii (I-B)) provided fermented milk with a particularly mild flavor.

(2) FIG. 24 illustrates the relationship between the first principal component score (PC1) and the second principal component score (PC2) in various types of fermented milk, which was obtained by performing principal component analysis (PCA) on the sensory evaluation values of the fermented milk obtained by combinations (Y16, Y36, Y6K(−), Y6K(+)) of various S. thermophilus with L. delbrueckii NITE ABP-02874 being L. delbrueckii classified into subcluster I-B of cluster I or L. delbrueckii 2038 being L. delbrueckii classified into cluster IV. In addition, FIG. 25 illustrates a scatter diagram of the sensory evaluation items corresponding to FIG. 24 .

As illustrated in FIG. 24 and FIG. 25 , the fermented milk (Y16) using S. thermophilus prtS(−) in combination with L. delbrueckii (IV) (L. delbrueckii 2038) had a strong acidity, and the fermented milk (Y6K(−)) using a combination with L. delbrueckii (I-B) (L. delbrueckii NITE ABP-02874) had a weak umami and sweetness, where there was no significant difference from the flavor of milk before fermentation, and the flavor as fermented milk tended to be poor. Meanwhile, the fermented milk (Y36) using S. thermophilus prtS(+) in combination with L. delbrueckii (IV) (L. delbrueckii 2038) tended to have a strong astringency/unpleasantness and umami. In contrast, the fermented milk (Y6K(+)) using S. thermophilus prtS(+) in combination with L. delbrueckii (I-B) (L. delbrueckii NITE ABP-02874) tended to have a strong sweetness and yogurtness, and was evaluated as having a well-balanced mild flavor as a whole. In addition, it was confirmed that the fermented milk was clearly differentiated in flavor from fermented milk produced by other combinations of strains.

<Measurement of Lactic Acid Levels (in the Fermented Milk)>

In the above fermentation time measurement, the amounts of D-lactic acid and L-lactic acid in the fermented milk obtained using various combinations of strains presented in Table 6 were measured by high performance liquid chromatography (HPLC) in accordance with the following method.

(Measurement Method)

The fermented milk was diluted 2-fold with ultrapure water and deproteinized using the Carrez reagent. The supernatant after the deproteinization was filtered through a filter vial (PVDF, 0.2 μm, 1030-19022, manufactured by THOMSON), and the amounts of D-lactic acid and L-lactic acid (mM) in each type of fermented milk were measured under the following conditions:

-   -   Guard column: SUMICHIRAL OA-5000 5 μm 4 mm i.d.×10 mm         (manufactured by Sumika Chemical Analysis Service, Ltd.)     -   Column: SUMICHIRAL OA-5000 4.6 mm i.d.×150 mm (manufactured by         Sumika Chemical Analysis Service, Ltd.)     -   Oven temperature: 40° C.     -   Flow rate: 1.0 mL/min     -   Detector: SPD-M20A (manufactured by Shimadzu Corporation)     -   Injection volume: 10 μl     -   Mobile phase: 2 mM CuSO₄·5H₂O+5% isopropanol         (Results)

(1) FIG. 26 and FIG. 27 illustrate the relationship between the amounts of D-lactic acid and L-lactic acid (mM) in various types of fermented milk and the acidity score in the above-described sensory evaluation (the acidity score, the greater the score, the stronger the acidity). In addition, the ratio of D-lactic acid to the total amount of lactic acid in the fermented milk ({D-lactic acid amount/(D-lactic acid amount+L-lactic acid amount)}×100(%)) was calculated. FIG. 28 illustrates the relationship between the obtained ratio of D-lactic acid and the acidity score. As illustrated in FIGS. 26 to 28 , the acidity score and the amount of D-lactic acid exhibited a positive correlation, and the acidity score and the amount of L-lactic acid exhibited a negative correlation. In addition, it was revealed that the higher the ratio of D-lactic acid, the higher the acidity score. From this, it was confirmed that as the ratio of D-lactic acid was smaller, the fermented milk had a lower acidity, that is, a milder flavor.

(2) FIG. 29 illustrates a graph comparing the ratio of D-lactic acid in the fermented milk for each S. thermophilus. Specifically, the figure illustrates the ratios of D-lactic acid (total) in the fermented milk (average values, %) when using S. thermophilus prtS(−) and L. delbrueckii (prtS(−): Y1 to Y20, Y6K(−), n=21) and when using S. thermophilus prtS(+) and L. delbrueckii (prtS(+): Y21 to Y40, Y6K(+), n=21).

In addition, FIG. 30 illustrates a graph comparing the ratio of D-lactic acid in the fermented milk obtained by combining various S. thermophilus with L. delbrueckii classified into a cluster except for IV (L. delbrueckii (except for IV)). Specifically, the figure illustrates the ratios of D-lactic acid (except for IV) in the fermented milk (average values, %) when using S. thermophilus prtS(−) and L. delbrueckii (except for IV) (prtS(−): Y1 to Y15, Y19 to Y40, Y6K(−), n=18) and when using S. thermophilus prtS(+) and L. delbrueckii (except for IV) (prtS(+): Y21 to Y35, Y39-Y40, Y6K(+), n=18).

As illustrated in FIG. 29 , for each S. thermophilus, the comparison of the ratio of D-lactic acid in fermented milk was made on L. delbrueckii classified into clusters I to V, but no significant difference was recognized. On the other hand, as illustrated in FIG. 30 , in the case of L. delbrueckii (except for IV), that is, L. delbrueckii classified into clusters I, II, III, and V, the ratio of D-lactic acid in fermented milk using S. thermophilus prtS(+) in combination was significantly lower than that in the fermented milk using S. thermophilus prtS(−) in combination (P-value by t-test was less than 0.05). Thus, it was confirmed that, when combined with S. thermophilus prtS (+), the fermented milk had a lower acidity, that is, a milder flavor.

<Examination on S. Thermophilus Strains>

The effects in the case of using strains of S. thermophilus except for S. thermophilus NITE ABP-02875 described above were examined. The examination was carried out by using the strains of S. thermophilus carrying no prtS gene (S. thermophilus prtS (−):MEP1900415, MEP1900416, MEP1900417, and MEP1900418) and the strains of S. thermophilus carrying a prtS gene (S. thermophilus prtS(+): MEP1900419, MEP1900420, MEP1900421, and MEP1900422), both of which had been confirmed for the presence or absence of prtS gene carried. All of these strains assigned numbers beginning with “MEP” are stored by Meiji Innovation Center of MeijiCo., Ltd. (postal code: 192-0919, 1-29-1 Nanakuni, Hachioji-shi, Tokyo, Japan).

(1) The above-described various S. thermophilus prtS(−) and L. delbrueckii NITE ABP-02874 were used in combination to measure the time required for fermentation. Specifically, each strain was subjected to activation culture twice at 37° C. for 16 hours in a 10% skimmed milk medium (sterilized at 121° C. for 7 minutes) prepared with 10% (wt/wt) skimmed milk powder and 0.1% (wt/wt) yeast extract. These strains were combined and added to a 10% skimmed milk medium (sterilized until reaching at 95° C.) to a total concentration of 1% (L. delbrueckii:S. thermophilus=about 1:1 (bacterial count)), followed by fermentation at 43° C. for 24 hours (aerobic, static culture) to obtain various types of fermented milk. The time from the addition of each strain until the pH reached 4.5 was measured and defined as the fermentation time required for fermentation. In addition, the time required for fermentation was measured in the same manner as described above, using combinations of the above-described various S. thermophilus prtS(+) and L. delbrueckii NITE ABP-02874. FIG. 31 illustrates fermentation times (average values, minutes) when using S. thermophilus prtS(−) (prtS(−), n=4) and using S. thermophilus prtS (+) (prtS (+), n=4). As a result of the Mann-Whitney U test, the result for prtS (+) was such that the P-value was less than 0.05 for the result of prtS(−). As illustrated in FIG. 31 , it was confirmed that, also in the case of using S. thermophilus prtS(+) except for S. thermophilus NITE ABP-02875, the fermentation time was significantly reduced as compared with the case of using S. thermophilus prtS(−), as in the case of using S. thermophilus NITE ABP-02875.

(2) The ratio of D-lactic acid in the fermented milk obtained by using the combination of the various S. thermophilus prtS (−) and L. delbrueckii NITE ABP-02874 of (1) above was measured and calculated in the same manner as in the above-described measurement of the amount of lactic acid. In addition, the ratio of D-lactic acid in the fermented milk obtained by using the combination of the various S. thermophilus prtS(+) and L. delbrueckii NITE ABP-02874 of (1) above was measured and calculated in the same manner as in the above-described measurement of the amount of lactic acid. FIG. 32 illustrates the ratios (average values, %) of D-lactic acid when using S. thermophilus prtS(−) (prtS(−), n=4) and when using S. thermophilus prtS (+) (prtS (+), n=4). As a result of the t-test, the result for prtS(+) was such that the P-value was less than 0.05 for the result of prtS(−). As illustrated in FIG. 32 , it was confirmed that, also in the fermented milk obtained by using S. thermophilus prtS(+) except for S. thermophilus NITE ABP-02875, the ratio of D-lactic acid was significantly reduced as compared with the fermented milk obtained by using S. thermophilus prtS(−), as in the case of using S. thermophilus NITE ABP-02875, and the fermented milk had a lower acidity and milder flavor.

As has been described above, the present invention makes it possible to provide a method for producing fermented milk using a Lactobacillus species and Streptococcus thermophilus, a lactic acid bacterium and a lactic acid bacteria-containing composition which can be used therein, and fermented milk obtained therefrom. The method for producing fermented milk makes it possible to produce fermented milk even when using Lactobacillus delbrueckii which has no lactose utilization and to obtain fermented milk having a balanced and mild flavor in all of acidity, umami, fermented aroma, and richness. 

What is claimed is:
 1. A combined product comprising: a Lactobacillus delbrueckii and a Streptococcus thermophilus carrying a prtS gene, wherein the Lactobacillus delbrueckii is Lactobacillus delbrueckii classified into any one of clusters I, II, III, and V by multilocus sequence analysis (MLSA) classification based on seven housekeeping genes consisting of fusA gene, gyrB gene, hsp60 gene, ileS gene, pyrG gene, recA gene, and recG gene.
 2. The combined product according to claim 1, wherein the Streptococcus thermophilus carrying the prtS gene is Streptococcus thermophilus specified by accession number NITE BP-02875.
 3. The combined product according to claim 1, wherein the Lactobacillus delbrueckii is satisfying all of the following conditions (i) to (vii): (i) carrying an fusA gene whose allele number is 2 or 14, (ii) carrying a gyrB gene whose allele number is any one selected from 3, 17, 18, and 25, (iii) carrying an hsp60 gene whose allele number is any one selected from 4, 18, 26, and 27, (iv) carrying an ileS gene whose allele number is any one selected from 16, 20, 21, 30, and 33, (v) carrying a pyrG gene whose allele number is 22 or 23, (vi) carrying an recA gene whose allele number is any one selected from 2, 7, 24, and 28, and (vii) carrying an recG gene whose allele number is 3, or the Lactobacillus delbrueckii is classified into the same cluster as that of the Lactobacillus delbrueckii satisfying all of the conditions (i) to (vii) by MLSA classification based on seven housekeeping genes consisting of fusA gene, gyrB gene, hsp60 gene, ileS gene, pyrG gene, recA gene, and recG gene.
 4. The combined product according to claim 1, wherein the Lactobacillus delbrueckii is specified by accession number NITE BP-02874, or the Lactobacillus delbrueckii is classified into the same cluster as that of the Lactobacillus delbrueckii specified by accession number NITE BP-02874 by MLSA classification based on seven housekeeping genes consisting of fusA gene, gyrB gene, hsp60 gene, ileS gene, pyrG gene, recA gene, and recG gene.
 5. The combined product according to claim 1, wherein the Lactobacillus delbrueckii is Lactobacillus delbrueckii specified by accession number NITE BP-02874.
 6. The combined product according to claim 1, wherein the Lactobacillus delbrueckii has no lactose utilization. 